Recent developments in the area of wireless communications have introduced the implementation of multiple antennas in a mobile subscriber unit as well as in a base station (or equivalent access point to a communication network) serving the mobile subscriber unit. The use of multiple antennas in the mobile subscriber unit enables “multiple input multiple output” (MIMO) communication between the mobile subscriber unit and the serving base station. MIMO communication has been introduced into wireless communications in order to improve the spectral efficiency of communication. In environments where there is no line-of-sight between the mobile subscriber unit and the base station and where there exist obstacles from which transmitted signals reflect, different signal paths between transmitter and receiver antennas may be seen as sub-channels which have different characteristics. Depending on the environment, the sub-channels may be more or less correlated. The characteristics of the sub-channels may be utilized in many ways in order to improve the data throughput and/or reliability of data transmission.
A power source (typically a battery) of the mobile subscriber unit has a limited amount of power and, therefore, power consumption of the mobile subscriber unit should be minimized. Characteristics of a transmitted signal and, particularly, the peak-to-average power ratio (PAR) of the transmitted signal typically have a significant effect on the efficiency of an amplifier and, consequently, on the power consumption of the mobile subscriber unit. High PAR sets high requirements on the radio frequency (RF) amplifier of the mobile subscriber unit and, thus, the efficiency of the RF amplifier reduces, resulting in increased power consumption. High PAR usually results from the summation of signals.
As mentioned above, the characteristics of the sub-channels may be utilized in many ways in order to improve the data throughput and/or reliability of data transmission. Efficient solutions in the data throughput point of view are schemes known as “waterfilling” transmission schemes. In short, waterfilling refers to transmitting more data into high-quality sub-channels and less data into low-quality sub-channels. The transmission of data into a sub-channel comprises the transmission of a signal from several transmitter antennas. When data is transmitted into several sub-channels, signals to be transmitted from each antenna are summed, resulting in a high PAR of the signal transmitted from one antenna and in increased power consumption.